For the record, I don't think hiring Meyer or Riley is necessarily a better hire than a guy like Campbell, and I'd argue that I'd rather have Zimmer than either of them.
Sure, they're sexier/more exciting names, college success is enticing and Riley certainly looks like he has a "McVay"-type of offensive mind, but the fact of the matter is that there is a huge learning curve going from college to the NFL and even the best college HC's often struggle with the transition. I'd be willing to see what happens with either of them just because I'm willing to try anything but Garrett at this point, but hiring either of them would be a massive risk.
There is a very credible argument that Campbell (or another top-flight assistant like Roman, etc.) and certainly Zimmer would be better hires.